Fix ChangelogBackendTestCase randomness

ChangelogBackendTestCase.java:
Introduced assertLastCookieIsEqualTo() to do an equivalent to "wait until keyword suceeds"
In searchInCookieModeOnTwoSuffixesWithPrivateBackend(), do not forget to close the domain.
In isOldestCSNForReplica(), fixed error message.
Overrode getGenerationId() to have a fixed generationId. This ensures messages can always sent from ReplicationBroker to LDAPReplicationDomain and processed.